The performance of thin film solar cells depends on the absorption of incident light. In general, one may expect that reflection from the outermost surface of the cell limits the amount of incident solar energy entering the cell. A further consideration is that the absorbing semiconductor layers are too thin to absorb all of the usable solar energy in one optical pass. This is a consequence of limited minority carrier diffusion length in thin film semiconductor absorber materials. In the specific case of front-wall CdS/Cu.sub.2 S cells, for example, which are built-up sequentially by the deposition of CdS and Cu.sub.2 S on a zinc plated copper substrate, the substrate has been found to have an influence of up to 25% on the efficiency of the cells. Experience with the CdS/Cu.sub.2 S cell and a general theoretical analysis have shown that the substrate must have high reflectivity in the spectral region about 500 nm. In addition, the substrate should be a diffuse reflector in a specific way. The diffuseness should be such that normally incident radiation is reflected at such an angle that on re-entering the absorbing semiconductor layer, the light is interally reflected and trapped within the cell.
Two other techniques are well known for achieving high absorption of incident light in solar cells. The first is to use antireflection layers in order to reduce reflection of incident light from the outermost surface of the cell. The second technique, set forth by A. B. Tanos in U.S. Pat. No. 3,480,473, is to roughen the front surface of the cell by etching so that incident light undergoes multiple reflections. Present high efficiency thin film CdS/Cu.sub.2 S solar cells described in the IEEE Transactions on Electron Devices, Vol. ED-27, No. 4, pages 645-51, April, 1980, used a single layer silicon oxide antireflection coating, and etched CdS to form the cell built on a reflecting substrate. The substrate was formed by electroplating a thin layer of zinc onto the matte side of electroformed copper foil of the type used, for example, in the printed circuit board manufacture art. As a result of the electroforming process, the matte side of the copper foil is characterized by pyramidal protuberances which are, on the average, three microns high from their base to their peak and are spaced eleven microns apart over the entire surface. This roughness produces the desired diffuse reflections. High reflectivity is obtained as a result of reaction between the zinc layer and the underlying copper to form brass, which has substantially higher reflectivity than zinc.
However, it is also known to the art that increased area of the CdS/Cu.sub.2 S heterojunction reduced the achievable open circuit voltage. The increased area is the result of roughness of the substrate and etching described above.
Roughened surfaces can also diminish the efficiency of solar cells having grid lines for transparent contacts. The electrical path length of a grid line traversing a roughened surface is greater than across a smooth surface, causing resistive power losses. In the prior art pertaining to roughened cells, thicker grid lines were employed to compensate for increased length. Since many thin film solar cells use noble metals for the transparent contact, this approach is economically disadvantageous.
A further consideration is that for solar cells to be an economically attractive energy source, quite thin (less than ten micron) layers are desirable. The previously useful substrate roughness known to the art is, therefore, disadvantageous for the formation of continuous, pinhole free films that are less than ten microns thick by vacuum evaporation techniques. It is also an advantage to low cost manufacture that the solar cell be built up sequentially by depositing the semiconductor layers on a flexible metal foil, the foil being capable of functioning as the substrate, opaque contact, and diffuse reflector.
Another consideration is reliability, since it is known that roughened surfaces react more rapidly with corrosive ambients. Roughened surfaces are also known to accumulate moisture and soil which diminish the efficiency of solar cells and ultimately lead to failure.